Application Of Pulsed Electric Fields On Milk Sterilization And Its Effect On Milk Fat

Pulsed electric field (PEF) treatment is one of the nonthermal processing technologies that can be applied to inactive spoilage and pathogenic microorganisms of liquid foods. Most of the current researchs focus on its effect on microorganisms, enzymes, and nutrient components of fruit and vegetable juices. There are limited reports about the influence of PEF on the lipid of food. In this study, the effect of PEF on milk fat and its mechanism were investigated.Firstly, lethal and sublethal injury of Escherichia coli, Staphylococcus aureus and Listeria monocytogenes in milk by PEF were determined and inactivation kinetics was analyzed. The proportion of damaged cells depends on the species of microorganisms, electric field strength and treatment time. S. aureus was the most resistant microorganism which showed the highest sublethal injury proportion. Moreover, PEF treatment at 30 kV/cm for 800μs was selected as the most effective treatment to inactive microorganisms, and the shelf life of milk was evaluated in comparison with pasteurized treatment (75℃,15s).Secondly, the effects of PEF treatments and storage time on the ascorbic acid content and antioxidant capacity of milk were evaluated and compared to pasteurized treatment. The ascorbic acid content decreased gradually with the increased electric field strength and treatment time (P<0.05), and the degradation follows a zero kinetics equation. PEF treatment with high strength resulted a significant decrease of antioxidant capacity of milk. During the shelf life of milk, the content of ascorbic acid and the antioxidant capacity of PEF-treated milk were higher than that in pasteurized milk, and the degradation of ascorbic acid follows a first kinetics equation.Thirdly, changes of milk flavor under PEF and pasteurized treatments were investigated. Total of thirty-seven volatile compounds were detected in raw, PEF-treated and pasteurized milk by GC-MS. Pasteurization resulted in an increase of aldehyde and ketone contents, while PEF treatments resulted in an increase of aldehyde contents alone.Finally, in order to investigate the mechanism of lipid oxidation under PEF treatment, oleic acid emulsion was selected to investigate radical species generated during PEF treatment. The ESR results showed that hydrogen radicals were formed in oleic acid emulsion under PEF treatment, and ascorbic acid has the ability to inhibit the hydrogen radicals. The reaction activity of hydrogen radicals and its effect on oxidation of milk fat were discussed.